As my relations with the various factions declined, the tech 'The Enemy on Earth' finally appeared. I completed that tech, and stunned several aliens since then. But the officially reported 'captures' don't add to my total in the 'Containment' screen. The tech tree tool provided indicates a need another autopsy to advance. Perhaps if I keep playing, and let some of the Alliances favor drop further, another race will show up besides the 4 I've seen for another autopsy to be done?
(Database reference: rs_alien_xvi_event)
I guess could figure this out by looking at the code, but hoping for the lazyman approach)